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How to measure small currents while in sleep mode

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Minority whip
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2010/09/22 16:17:12 (permalink)
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How to measure small currents while in sleep mode

Folks:

With so many low powered uC's running in sleep mode, I was wondering if anyone has had difficulty measuring sleep currents while in sleep mode. Especially interested if sleep mode is dynamic and not static. A DMM doesn't seem to have the resolution, and a scope has a noise floor interferring with the signal. I am trying to figure out my average current draw from of a battery. Any suggestions?

A....grin

The wife and I are like Beauty and The Beast. but if anyone calls her a beast, I'll murder' em'
#1

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    Ironic
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    Re:How to measure small currents while in sleep mode 2010/09/22 16:55:53 (permalink)
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    maybe a resistor in series with power line.

    try using a value that causes around 100 milivolts of drop for expected current value.
    Enough for measuring with digital voltmeter, and hopefully not enough to cause problems for pic operation.
    However, if it leaves sleep mode, current consumption and voltage drop will increase and there will be trouble...
    #2
    Steven37
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    Re:How to measure small currents while in sleep mode 2010/09/22 17:24:55 (permalink)
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    Hi,

    I agree with Ironic, measure the voltage across a small resister between the battery and the pic (or between battery and pic regulator if used).
    As Ironic said select the resistor to get a voltage drop of about 100mV.
    I = V/R, If the reading has a lot of noise a simple R/C low pass filter between the resistor and the multimeter will give a stable reading. (10K series resistor and 470uF across the multimeter)
    If using a regulator, the measurement between the battery and the regulator will also show how much quiescent current the regulator is using, unless you select a low quiescent regulator, the regulator may use much more current than a sleeping pic.

    Regards Steve.
    post edited by Steven37 - 2010/09/22 17:29:46
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    john
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    Re:How to measure small currents while in sleep mode 2010/09/23 17:31:58 (permalink)
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    I've done this:

    1) Use a decent meter with the appropriate scale.  As noted above, and in my experience as well, a resistor shunt measuring uA can introduce more troubles than its worth, IMHO. Lots of inexpensive DMMs have 200.0us scales.

    2) To fix the problem of pegging your uA meter AND dragging down Vdd due to a cheapo meter's relatively high shunt impedance when the PIC wakes up:
    2a) jumper the meter leads until the PIC sleeps and take your measurements then or..
    2b) add a high side switch, MOSFET or whatever fits, controlled by an output pin, across the meter (and its shunt) that opens when the PIC goes into sleep.  Note that the code must open the switch and sleep using power stored in the bus capacitors. Likewise, the caps must have enough to run the PIC coming out of sleep long enough to close the switch.  I use a build-time switch to install the extra code to control the switch and remove it for release versions.

    If you are tight on I/O, consider build-time reassignment of another IO line i.e. some LED that won't be used in power down, etc.


    post edited by john - 2010/09/23 17:54:12

    John
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    Brick
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    Re:How to measure small currents while in sleep mode 2010/09/23 17:52:37 (permalink)
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    I've doen this the resistor way, easiest way is to have a pot in series with the power. Have it set to zero ohms when its operating normally (programming ect) then get it to sleep.

    Once its sleeping ramp the resistance up till you get a decent voltage drop (as mentioned before around 100mv). Then measure the final resistance once you have turned it of and do the math :)
    #5
    mhanuel
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    Re:How to measure small currents while in sleep mode 2010/09/23 18:27:45 (permalink)
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    You could look for some IC that does the analog part, like ADM1192.
    The good part is that is very easy to use because it have an I2C interface. The bad thing is that you should make some PCB.
    If you are interested I can send you the artwork of a PCB I have done for general purpose applications using ADM1192.

    Best,

    #6
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